iphtheria in tropical climates, which are characterized by excessive
rainfall, and its greater prevalence in continental than in insular
countries, confirm his theory. His observations seem quite contrary to
the view laid down by various authorities, and hitherto accepted, that
wet weather favours diphtheria. The two, however, are not
irreconcilable. The key to the problem--and possibly to many other
epidemiological problems--may perhaps be found in the movements of the
subsoil water. It has been suggested by different observers, and
particularly by Mr M. A. Adams, who has for some years made a study of
the subsoil water at Maidstone, that there is a definite connexion
between it and diphtheria. In England the underground water normally
reaches its lowest level at the end of the summer; then it gradually
rises, fed by percolation from the winter rains, reaching a maximum
level about the end of March, after which it gradually sinks. This
maximum level Mr Adams calls the annual spring cleaning of the soil, and
his observations go to show that when the normal movement is arrested or
disturbed, diphtheria becomes active. Now that is what happens in
periods of drought. The underground water does not rise to its usual
level, and there is no spring cleaning. The hypothesis, then, is this:
The diphtheria bacillus lives in the soil, but is "drowned out" in wet
periods by the subsoil water. In droughty ones it lives and flourishes
in the warm, dry soil; then when rain comes, it is driven out with the
ground air into the houses. This process will continue for some time, so
that epidemic outbreaks may well seem to be associated with wet. But
they begin in drought, and are stopped by long-continued periods of
copious rainfall. This is quite in keeping with the observed fact that
diphtheria is a seasonal disease, always most prevalent in the last
quarter of the year. The summer develops the poison in the soil, the
autumnal rains bring it out. The fact that the same cause does not
produce the same effect in tropical countries may perhaps be explained
by the extreme violence of the alternations, which are too great to suit
this particular micro-organism, or possibly the regularity of the
rainfall prevents its development.

The foregoing hypothesis is supported by a good deal of evidence, and
notably by the concurrence of the great epidemic or pandemic prevalence
in Great Britain, culminating in 1859, with a prolonged period of
exceptio